The hydroxyphenylpyruvate dioxygenases (HPPDs) are enzymes that catalyze the reaction in which para-hydroxyphenylpyruvate (HPP) is transformed into homogentisate. This reaction takes place in the presence of enzyme-bound iron (Fe2+) and oxygen. Herbicides that act by inhibiting HPPD are well known, and include isoxazoles, diketonitriles, triketones, and pyrazolinates (Hawkes “Hydroxyphenylpyruvate Dioxygenase (HPPD)—The Herbicide Target.” In Modern Crop Protection Compounds. Eds. Krämer and Schirmer. Weinheim, Germany: Wiley-VCH, 2007. Ch. 4.2, pp. 211-220). Inhibition of HPPD blocks the biosynthesis of plastoquinone (PQ) from tyrosine. PQ is an essential cofactor in the biosynthesis of carotenoid pigments which are essential for photoprotection of the photosynthetic centres. HPPD-inhibiting herbicides are phloem-mobile bleachers which cause the light-exposed new meristems and leaves to emerge white. In the absence of carotenoids, chlorophyll is photo-destroyed and becomes itself an agent of photo-destruction via the photo-generation of singlet oxygen.
Methods for providing plants that are tolerant to HPPD herbicides are also known. These methods have included: 1) overexpressing the HPPD enzyme so as to produce quantities of HPPD enzyme in the plant that are sufficient in relation to a given herbicide so as to have enough of the functional enzyme available for the plant to thrive despite the presence of the herbicide; and 2) mutating a particular HPPD enzyme into an enzyme that is less sensitive to inhibition by herbicides. Methods for mutating HPPD enzymes for improved HPPD herbicide tolerance have been described (see, e.g., PCT Application Nos. WO 99/24585 and WO 2009/144079), and some particular mutations of plant HPPD enzymes (e.g., mutation of G422 in the Arabidopsis HPPD sequence) are purportedly capable of providing some measure of tolerance to mesotrione and other triketone herbicides. However, the enzyme kinetic and whole plant data reported thus far are insufficient to conclude whether the reported mutational changes confer commercially significant benefits over the corresponding wild type enzyme(s).
Furthermore, while a particular HPPD enzyme may provide a useful level of tolerance to some HPPD-inhibitor herbicides, the same HPPD may be quite inadequate to provide commercial levels of tolerance to a different, more desirable HPPD-inhibitor herbicide (See, e.g., U.S. Patent Application Publication No. 20040058427; PCT Publication Nos. WO 98/20144 and WO 02/46387; see also U.S. Patent Application Publication No. 20050246800 relating to the identification and labelling of soybean varieties as being relatively HPPD tolerant). Moreover, mutated versions of HPPDs from cool-climate grasses with improved resistance to triketone-type herbicides have yet to be reported. Such mutants would be highly desirable, as HPPDs from cool-climate grasses are preferable to other types (see, e.g., PCT Application No. WO 02/46387 and Hawkes et al. 2001 in Proc. Brit. Crop Prot. Conf. Weeds 2, 563). Accordingly, new methods and compositions for conferring commercial levels of HPPD herbicide tolerance upon various crops and crop varieties are needed.